Fabrication of flat panel displays is well known in the art. Flat panel displays may be comprised of active matrix or passive matrix panels. Active matrix panels and passive matrix panels may be configured as transmissive, reflective, transflective or emissive dislays. Liquid crystals, electroluminescent (EL) materials, organic light emitting diodes (OLEDs), or light emitting diodes (LEDs), to name a few, may be used in fabricating flat panel displays. Liquid crystal displays operate fairly reliably but their manufacturing process is complex and typically requires high power backlighting. OLED are among the currently favored display types due to their promise to deliver full color, flexible and inexpensive FPDs because of their simple manufacturing process.
OLEDs are electroluminescent (EL) devices that emit light generated by radiative recombination of injected electrons and holes within one or more organic EL layers of the OLEDs. OLEDs have electrical and optical characteristics which are attractive for operation within pixel-addressed displays. For example, OLEDs operate at low voltages and are relatively efficient in converting electrical current into light. In addition, OLEDs can be fabricated into thin, lightweight display devices. Furthermore, OLEDs can be designed to emit light of different colors to create color and multi-color display devices. Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays. Many of the thin organic films used in such devices are transparent in the visible spectral region.
Organic light emitting devices (OLEDs) comprise of several organic layers in which one of the layers comprises of an organic material that can be made to electroluminesce, by applying a voltage across the device. The organic material can be small molecule polymers or large molecule (conjugated) polymers that are electroluminescent. FIG. 1A illustrates a simple example of an OLEDs structure, an OLED display device 1 of a current state of the art. The OLED display device 1 includes a substrate 2, an anode layer 4, an organic electroluminescent (EL) region 6, and a cathode layer 8. It is also common that the EL region 6 comprises a hole transport layer (HTL) 6c, a light emitting polymer layer 6b, and an electron transport layer 6a. The substrate 2 may be transparent or opaque. Thus, the display device may be configured to emit light through the substrate, or through the cathode layer 8. When light is emitted from the organic molecules (e.g., EL region 6) through the substrate 2, the OLEDs are referred to as bottom emitting OLEDs. When light is emitted from the organic molecules (e.g., EL region 6) through the cathode layer 8, the OLEDs are referred to as top emitting OLEDs.
In the bottom emitting OLEDs, the substrate 2 is transparent and can be made of glass or plastic. In the top emitting OLEDs, the substrate can be opaque and be made of Si, plastic or a flexible metal foil; the anode layer 4 is typically made of a transparent conducting material, such as Indium Tin Oxide (ITO); and, the cathode layer 8 is typically made of a conducting metal with a low workfunction, such as Ca or Mg. The anode layer 4 and the cathode layer 8 are patterned, so that individual pixels of the display device can be addressed. The organic EL region 6 is composed of at least one organic or polymer layer.
Although the relative positions of the anode and cathode layers 4 and 8 may be inverted, such structures yield devices with very low efficiency. Thus, conventional OLED display devices have typically been configured with the anode layer 4 located between the cathode layer 8 and the substrate 2, as illustrated in FIG. 1A. The reason for this preference is that the cathode layer is a more efficient injector of electrons when deposited on organic layer, e.g., when it is the last deposited layer of the stack.
LEDs and OLEDs are similar in that both the LEDs and the OLEDs emit light when electrical current passes through the device (diode) except that OLEDs use organic material in the formation of the diode of the LEDs.
The current state of the art in fabricating organic light emitting diode structures is using one substrate on which all the layers (including addressing buses are deposited). The OLED structure is then encapsulated or capped with a lid having either a periphery seal or globally distributed adhesive.
Under the current art, depositing light emitting polymers onto the substrate on which all of the layers are to be deposited subjects the finished product to poor quality problem. FIG. 1C illustrates an exemplary active matrix 20 made with OLED using the current technology. The active matrix 20 includes a substrate 32, a plurality of thin film transistors 30 made from silicon materials formed on top of substrate 32. An ITO layer 34 (an anode) is deposited between each thin film transistors 30 and is electrically connected to the thin film transistors 30. An HTL layer 36 is deposited on top of the ITO layer 34. A thin layer of light emitting polymer 37 is deposited on top of an HTL layer 36. A cathode electrode layer 38 is then deposited on the light emitting polymer layer 37. Under the current art, only a limited number of transistors (e.g., 2-4 transistors) per pixel area can be formed in this region. This limitation limits the options for driving the display medium, which in turn contributes to brightness nonuniformity typically seen in these types of active matrix displays.
Another way of making an active matrix display is illustrated by way of an example in FIG. 1B. FIG. 1B illustrates a polymer dispersed liquid crystal display 10 (PDLC). The PDLC display 10 includes a substrate 12, integrated circuits 14, planarization layer 16, a metalization layer 18, a PDLC display medium 22, an ITO layer 24, and backing layer 26. The metalization layer 18 connects one integrated circuit 14 to another and also forms a patterned electrode to define an active pixel area. When applying this configuration to making an OLED display, one problem with this configuration is that more complex planarization is required since the OLED display medium is usually too thin resulting in defects if the topography of the surface is not perfectly planarized. In addition, the metalization layer 18 must be replaced by ITO for the most common configuration of a bottom emitting display.